Not sure if this would fit with the game
Basically just add a secret room or a room with a puzzle to unlock it
behind it there’s a pedestal /shrine to looks ancient with completely fictional tech
This could be a boss fight / new weapon / blessing
Now just some ideas
For the boss fight it could be an ancient deity that lost to the beast and got sealed by it
after it deems worthy you’d be awarded with a ancient weapon or a blessing
Weapon:
Just a way to expand the possibilities my ideas would have been
Wand / staff
Magic rifle that recharges with kills
Blessing:
A way to get specific build faster
Allows you to keep one relic for every run
This could be limited to some to make it balanced